PSI - Issue 72

Niki Tsivouraki et al. / Procedia Structural Integrity 72 (2025) 141–148

143

Fig. 1. The experimental procedure for the residual fatigue life and strength assessment.

2.3. Experimental damage index Regarding the experimental damage index, raw C-Scan images are utilized, with the delamination area considered as the sole damage mode. For each fatigue state, amplitude and time-of-flight images are selected, and an algorithm is implemented to analyze each pixel of the images. The algorithm assigns a degradation percentage to each pixel based on the colormap of the C-Scan, subsequently summing the values for each fatigue state (FS). The Damage Index (DI) is then calculated using Eq. (1), with Eqs. (2) and (3) explaining the individual ratios, whose values are shown in Fig. 2. Further details on the experimental C-Scan damage index can be found in Tsivouraki et al. (2024). The entire implementation is carried out using Matlab, where an increase in DI value corresponds to an increase in the delamination area.

1

(1)

DI



C Scan 

  

dm R W

davg

R

W

davg

dm

A A

b s

(2)

R



TOF TOF

1 2

(3)

W



where DI C-Scan i s

the damage index from the C-Scan, R dm is the amplitude ratio of the i th fatigue damage state, R davg is

the amplitude ratio of the healthy state, W davg is the time-of-flight ratio of the healthy state, W dm is the time-of-flight ratio of the i th fatigue damage state, A b is the amplitude value of the i th fatigue damage state, A s is the amplitude value of the healthy state, TOF 1 is the time-of-flight value of the healthy state and TOF 2 is the time-of-flight value of the i th fatigue damage state.

Fig. 2. Typical A-Scan signal from the immersion C-Scan tests.